Classes start at 9:00 every day. It's 9: 30 and the school bus is late.
When the bus gets to school, classes? has began_. The teachers _have been working_since 9: 00.
一般现在时叙述故事， 校车到时，已经开始上课了。过去的点已经开始 用现在完成时， 老师们从九点九开始工作，老师们从过去的时间点开始，现在还在进行并还会持续下去 所以用现在完成进行时。参考一下图示。